Iowa Code § 321.432: Horns and warning devices.

Every motor vehicle when operated upon a highway shall be equipped with a horn in good working order and capable of emitting sound audible under normal conditions from a distance of not less than two hundred feet, but no horn or other warning device shall emit an unreasonably loud or harsh sound or a whistle. The driver of a motor vehicle shall when reasonably necessary to ensure safe operation give audible warning with the horn but shall not otherwise use such horn when upon a highway.